(Bloomberg)—Smart cars will become delivery boxes for Deutsche Post AG’s DHL package operation in Germany this year in the largest test so far of the mail and auto industry working together to leverage connected cars.

Owners of Daimler AG’s Smart models can arrange for DHL to deliver parcels to the trunks of their parked cars starting in September in the parent company’s hometown of Stuttgart, with the service eventually rolled out to a total of seven cities in the following months, including Cologne and Berlin. The project, dubbed Smart Ready to Drop, will be the country’s biggest trial yet of in-car delivery for ordinary vehicle owners, with several hundred customers targeted in each city, Daimler said Monday in a statement.

“We’re planning many other services designed to ease urban life,” Annette Winkler, head of Smart, said in the statement.

Missed deliveries don’t just annoy shoppers, they’re a profit-sapping cost burden to package handlers such as Deutsche Post, Europe’s biggest mail operator. Similar efforts to use cars as delivery sites include a joint test last year by DHL, online retailer Amazon.com Inc., No. 1 in the Internet Retailer 2016 Top 500 Guide and 2016 Europe 500; and Volkswagen AG’s Audi luxury-car division and a Volvo Car Group project in Sweden. Amazon is developing drones to carry products to customers, while German retailer Metro AG is participating in trials of self-driving delivery robots.

Smart Ready to Drop will be available on models equipped with so-called connectivity box detectors that will become standard equipment as of September. The customer will use a mobile application to agree on delivery details for a car parked close to the recipient’s address. A code will enable the DHL courier to open the vehicle once during a specified timeframe to place the goods in the back, and to pick up any items being returned.

DHL is forecasting 5% to 7% average annual growth in parcel volume through 2020, with e-commerce a key demand driver. Atlanta-based competitor United Parcel Service Inc., which traditionally focused on carrying items between business customers, is forecasting that home deliveries will account for more than half its volume by 2019.